1. Benthic Fauna

2. Benthic Fauna Extremely diverse group of organisms
Many different groups, difficult to generalize about patterns

3. Distribution and Abundance
Limited by few general characteristics
Food availability
Type of substrate
Few phys/chem factors, especially dissolved oxygen

4. Littoral vs. Profundal
Benthic animals living in littoral region more varied than those in profundal region
Reflection of:
1) abundance of microhabitats
2) less stressful living conditions

5. Littoral Benthos
Protozoans, sponges, coelenterates, rotifers, nematodes, bryozoans, decapods, ostracods, cladocerans, copepods, bivalves, snails, insects, leeches

6. Littoral Benthos Microbenthos - very tiny
Vastly outnumber macrobenthos, and may contribute up to 50% of benthic production

7. Littoral Benthos Seldom food limited
Proximity to phytoplankton, macrophytes

8. Sublittoral Benthos Boundary between littoral & profundal
Species diversity drops off sharply
Mussels, ostracods, copepods, cladocerans from littoral - few typical dwellers

9. Profundal Benthos Very poor diversity Oxygen limited Other stressors:
Colder
Lower pH
Higher CO2, CH4, organics, P, NH3
Few can survive under these conditions

10. Profundal Benthos
Profundal benthos in eutrophic lakes resemble those of grossly polluted systems
Low diversity, monotony - great number of individuals, but only 1 or 2 species represented

11. Typical Profundal Assemblage
Chironomus midge larvae
Hemoglobin picks up limited oxygen
May also be able to use anaerobic respiration & excrete products of this process

12. Typical Profundal Assemblage
Oligochaete worms: Tubifex, Limnodrilus
Bury heads in organic sediments, wave tails with gills
Can develop huge populations (10,000+/m2)
Preyed on heavily by predaceous Chironomus

13. Typical Profundal Assemblage
Fingernail clams in genus Pisidium
Become dormant during anaerobic periods
May also inhabit temporary ponds

14. Typical Profundal Assemblage
Phantom midge larvae - Chaoborus
Temporary occupants - spend day on sediments, migrate into water column at night to prey on zooplankton
Not very tolerant of anoxia

15. Typical Profundal Assemblage
Also several microscopic forms that tolerate low oxygen
Some protozoan ciliates and flagellates, some nematodes

16. General Standing Crop
Most lakes have profundal benthos that averages ~5 g wet weight/m2
~1/2 g dry weight/m2

17. Less Productive Lakes
Profundal benthos more diverse in less productive (oligotrophic) lakes
Major reason: oxygenated sediment-water interface

18. Less Productive Lakes
More species of midge larvae, oligochaetes, immature insects like mayflies
Hexagenia - burrowing mayfly

19. Less Productive Lakes
Also more crustaceans like the amphipod Pontoporeia
Makes vertical migrations up to metalimnion at night (doesn’t eat zooplankton)

20. General Benthos Pattern
Diverse group in heterogenous, oxygenated littoral zone
Less diversity in more homogeneous profundal zone (less in more productive lakes)

21. Maxima of Abundance Two maxima - one in littoral - one in profundal
biomass
depth

22. Maxima of Abundance As systems become more productive, zone of
max. production shifts
from littoral to profundal,
then declines in
profundal
biomass
depth
Midges replaced
by oligochaetes

23. Seasonal Abundance Patterns
Lowest in summer (especially in insect-dominated communities)
Emergence of adults, high predation
Maximum densities and growth typically in autumn and winter in temperate zone

24. Predation by Fish
Predation can drastically reduce invertebrate standing crop
May be >50% of populations in some littoral areas
Predation losses in profundal areas generally much lower

25. Predation by Fish
Despite intense predation pressure, benthos dynamics and production mostly controlled by food supply